Method for the production of metal powders



METHOD FOR THE PRODUCTION OF METAL POWDERS Sven Sigvard Bergh, Vargon, Curt Anders Gustav Ericsson, Surahammar, and Biilge Troberg, Vargon, Sweden,

*fl P311311:

assignors to Wargons Aktiebolag, Vargon, Sweden, :1

company of Sweden N0 Drawing. Application September 17, 1953 Serial No. 380,862

Claims priority, application Sweden September 25, 1952 2 Claims. (Cl. 75--.5)

The present invention relates to a method for the production of powders of low carbon chromium alloys, particularly low carbon ferrochromium, and relates more particularly to a special correlation of the composition and the heat treatment for facilitating the crushing and grinding to a powder of a desired grain size.

The invention relates further to the use of such powders in Welding electrodes.

It has been previously suggested to manufacture powder by crushing and grinding, for instance, ferrochromium afiin, which means ferrochromium of the following composition:

Cr=6573%, C max=0.20%, Si=0.52.0%

the remainder being principally iron.

The alloy is practically free from injurious impurities.

It has proved, however, that crushing and grinding of this alloy is disproportionately expensive, which has hitherto resulted in a very limited use of such powders. If the costs for the crushing could be materially reduced, it would directly bring about a considerably extended use of powders of low carbon chromium alloys, forinstance as an alloy addition to the coatings for welding electrodes, as starting material for the manufacture of chromium com-pounds by the wet method and as addition in different metallurgical processes.

The solution of the problem of facilitating the crushing and grinding of low carbon chromium alloys has now been found to consist in the utilization of a phenomenon involving the formation of a so-called sigma-phase, occurring in certain stainless steels. This phenomenon, which has not hitherto been fully clarified, occurs on heating certain stainless or refractory chromium steels for a certain time to temperatureof about 700 C., and effects a considerable increase in the brittleness of the alloys thus treated. The same phenomenon has also been found in more highly alloyed chromium-iron alloys. According to an article by J. I. Heyer in Metal Progress, August 1951, pp. 5561, the range for the pure sigmaphase would lie between 42 and 52% chromium. Outside of this range there are ranges of mixed phases, coning to the invention for facilitating the crushing and the grinding.

Based upon the facts stated above, the method according to the invention is substantially characterized in that as starting material there is chosen an iron chromium alloy the composition of which is adapted to the formation of the sigma-phase, and that the molten alloy is ice cooled under such conditions, or the cooled alloy is subjected to such a heat treatment, that sigma-phase is formed, which increases the brittleness, whereupon the alloy is cooled to room temperature and crushed and ground by crushing and grinding methods and apparatus known per se. As a rule the alloys used as starting material should not contain more than 60% chromium, if it is not possible by an addition of other alloying elements to cause the formation of a sigma-phase which usually does not occur in such high chromium alloys. On the other hand, the chromium contents should not be below about 20%, as the formation of the sigma-phase is insuflicient at this low chromium content, and should usually be at least 30%. I

The fact that the powder manufactured according to the invention is hard and brittle is no drawback since on using the powder, e. g. in welding electrodes, it will be remelted, and simultaneously diluted, e. g. with iron, to such an extent that the sensitiveness to the formation of the sigma-phase and also the brittleness disappears.

The sigma-phase transformation is highly effected by the treatment temperature. Such a transformation'can be effected at from about 600 C up to about 880 C.

and heat treatments according to the invention can becarried out in all of this range, but we prefer to perform 7 the heat treatment in the upper part of said temperature range, e. g. at about 800 C., for attaining a faster transformation to the sigma-phase.

As appears from the above it is not necessary to heat the chromium alloys up to a temperature within the range between 880 and 600 C. On cooling the alloy from the molten state, i. e. from a temperature above 880 C., it will in most cases be sufficient to have the temperature drop slowly in said temperature range of 880-600 C. Such a cooling is also to be preferred to annealing for the reason that no special annealing furnace need be furnished, the treatment thereby being much less expensive.

Earlier investigations have also explained the favorable influence of certain additional elements upon the formation of the sigma-phase. The most important of the elements which promote the formation of the sigma-phase is silicon which, as a rule, is contained in the alloys comprised by this invention. Investigations which have been made show that a content of at least 1% silicon is advantageous for obtaining a sufiiciently rapid increase in brittleness. Other elements, which also promote the formation of the sigma-phase, and which, in combination with silicon, may be used alone or in combination with each other, are nickel, titanium, zirconium, niobium, tantalum, vanadium, molybdenum, tungsten, manganese, copper and aluminum. Some of these elements, e. g. nickel, manganese, copper, molybdenum, tungsten and niobium, can advantageously be added in greater quantities if the pulverized alloy is to be used for the manufacture of welding electrodes or the production of sintered stainless steel parts. For certain such complex compositions of the alloy no heat treatment or particularly slow cooling is required.

Aside from the improved crushability several other properties (described below) are obtained which are desirable especially in the manufacture of electrodes:

(1) Some of the added elements, e. g. nickel and copper, are ductile and it is consequently diflicult or impossible to crush them in the pure state. As a part in the chromium alloy they can easily be crushed.

(2) Since during the melting the added elements have been dissolved in molecular form in the alloy, each particle of the powdered alloy, obtained by the crushing, is of the same chemical composition. The special homogenizing or mixing treatment, which is otherwise necessary on mixing powders for the manufacture of weld-' 3 ing electrodes, is avoided. On storing and transporting the powder, no separation owing to differences in specific weight, grain, shape etc., can occur.

(3) Owing to the ternary,-'quaternary or still more complex composition of the alloy, its melting point is generally essentially lowercompared to that of the binary alloy. This is of very great importance when the alloy is used for the manufacture of welding electrodes and particularly when the electrode is'to contain powders of metals melting at high temperature, e. g., molybdenum and tungsten. When using the electrode the powder, the core material and possible fluxing agents are melted in the arc, and the melts are mixed in a short time, a low melting point (and a previous homogenizing treatment) being of the greatest importance.

As appears from the description above, one of the most important objects of the invention is to provide such powders which are used as alloying additions to welding electrodes of the type either consisting of a core of iron or iron alloy, surrounded by a slag forming coating, or consisting of a tube or a pipe filled with slag forming constituents. Electrodes are previously known, in which the slag forming constituents are mixed with dilierent kinds of metal powders such as ferromanganese-and ferrosilicon, serving as deoxidants, and alloying additions: such as ferrochromium and ferromolybdenum and metals for increasing the so-called yield of the welding, i.-e. therelation between the weight of the weld materal and the weight of the consumed core or tube. By practicing the fundamental idea of the invention it is possible directly to manufacture uniform powders, which whenused for welding electrodes of the types stated herein, combine in themselves the properties of previously added deoxidants,'e, g. ferrosilicon and ferromanganese, and other alloying elements, mainly serving to be incorporated in the weld and to improve its properties in one way oranother, e. g., for increasing the resistance to corrosion or erosion, increasing the refractory properties, improving the hardness, tenacity, or the like. These properties. are obtained by suitable adapting the alloying elements in the powder, while considering the general rule that the composition shall be chosen so that a phase transformation can be obtained, which increases the brittleness.

Thus, by suitably increasing the contents of silicon. and manganese, it is possible to obtain a desired deoxidizing eifect without loss of more expensive alloying elements. Further, by suitably adapting the nobler alloying ingredients, such as chromium, nickel and to a less extent molybdenum, copper, tungsten-and carbide forming elements such as titanium, vanadium, niobium, tantalum and zirconium, it is possible to obtain such a composition of the powder that when included in a welding electrode with an unalloyed or alloyed core or case, on melting of the electrode there is produced a weld of corrosion-resisting or refractory properties and with a content of alloying elements which is at least as high as in the material, e. g. stainless steel, which is to be welded with said electrode, and consequently with at least as good properties as thismaterial. If, according to these directions, there is to be manufactured a powder for use in electrodes with a low alloyed or unalloyed core, and the electrodes are to give a material in the weld of the so-called l8/8-type, i. e. a material containing about 18% chromium and 8% nickel and, possibly also containing lower contents of other metals which improve the properties, such as manganese, molybdenum, copper, aluminum, tungsten and carbide forming elements and others, the alloy from which thepowder is manufactured according to the invention should suitably contain chromium and nickel ina ratio between 2:1 and 3:1. Thus, at a chromium content between.30 and 60%, thepnickel content should'lie between 10 and 30%. Preferably we utilize such alloys with a chromium content between 40 and 50% and a nickel contentbetween l3 andf25.%. At the contents of chromium and nickel stated here, the

silicon content should lie between about 1% and a content of 4%, above the arnount necessary for the deoxidation and the alloy may advantageously in addition contain molybdemum up to 5% or more. The carbon content ought to be as low as possible, preferably not exceeding 0.20%. With alloys of the composition stated here it has proved that even normal cooling in air in some cases is sufiiciently slow for effecting the brittleness that promotes the crushability, an additional heat treatment in the temperature range GOO-880 C. often being superfluous.

As examples of compositions of alloys of the kind referred to herein which become brittle upon cooling from the molten state and without a separate heat treatment, the following may be cited:

Table 1 Percent Percent Percent Percent Percent Cr Ni Mo 0 1. 89 49. 8 13. 2 3. 25 0.05 1. 59 48. 4 15. 7 0. 14 1. 77 47. 9 15. 7 3. 0. 05 1. 52 45. 6 18. 5 2. (L14 1. 66 43. 3 l7. 7 3. 0O 0. l0 1. 66 43. 9 l6. 6 3. 28 0. 11 1. 68 44. O 14. 8 3. 40 0. 05

.The rest 'in'each case consists of iron with the accessory elements usually contained therein.

When theinvention is to be practiced for the manufacture of powders of ferrochromium afiin, the chromium content should not exceed 60%. As examples of some iron-chromium alloys with which the invention'can be practiced and which require the heat treatment described, Table 2 gives some of the compositions:

Table 2 Or 0 Si 58. 7 0. 17 1. 20 53. 9 0. 0s 1. 14 47. 7 0. 12 1. 21 44. 7 0. 05 0. 94 57. 9 0. 11 1. a2 50. s 0. 15 1.70 47. 0 0. 16 1. 60 42. a o. 15 1. as

The'rest also here in each case consists of ironwith theaccessory elements usually contained therein.

As a rule it can be said that the necessity for heat treatment decreases with increasing contents of elements such as silicon, molybdenum, tungsten, titanium, vanadiumand nickel.

The invention is not limited with respect to the nature of the slag forming ingredient or the binder when the alloy powder is used in a welding electrode coating. The coating may be acid, neutral or basic.

We claim:

1. A method for the manufacture of powders from low carbon iron-chromium-nickel alloys containing'from 30% to 60% of chromium and from 10% to 30% of nickel, the weight of the nickel amounting to from onehalf to one-third of the weight of the chromium'which comprises subjecting such an alloy to a-temperature within the range from 600 C. to 880 C. for a period of timesuflicient to effect the transformation of said alloy into the sigma phase, and cooling and crushing said alloy.

'2'. 'A method as defined in claim 1 in which the alloy is slowly cooled from a temperature above said temperature range through said temperature range.

(References on following page) References Cited in tse file of this patent 2,407,862 Wulfi -9. Sept. 17, 1946 UNITED STATES PATENTS 2,597,701 B61161 May 20, 1952 Re. 22,452 Clements et a1 Mar. 7, 1944 OTHER REFERENCES 5 Stainless Iron and Steel, vol 1, pages 87 to 91. Edited 2,291,482 McLott July 28, 1942 a}; Pubhshed 1951 by Chapman and 2,354,727 Wulfi Aug. 1, 1944 

1. A METHOD FOR THE MANUFACTURE OF POWDERS FROM LOW CARBON IRON-CHROMIUM-NICKEL ALLOYS CONTAINING FROM 30% TO 60% OF CHROMIUM AND FROM 10% TO 30% OF NICKEL, THE WEIGHT OF THE NICKEL AMOUNTING TO FROM ONEHALF TO ONE-THIRD OF THE WEIGHT OF THE CHROMIUM WHICH COMPRISES SUBJECTING SUCH AN ALLOY TO A TEMPERATURE WITHIN THE RANGE FROM 600*C. TO 880*C. FOR A PERIOD OF TIME SUFFICIENT TO EFFECT THE TRANSFORMATION OF SAID ALLOY INTO THE SIGMA PHASE, AND COOLING AND CRUSHING SAID ALLOY. 